/usr/share/fish/completions/apt-extracttemplates.fish is in fish-common 2.2.0-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 | #apt-extracttemplates
complete -c apt-extracttemplates -s h -l help --description "Display help and exit"
complete -r -c apt-extracttemplates -s t --description "Set temp dir"
complete -r -c apt-extracttemplates -s c --description "Specifiy config file"
complete -r -c apt-extracttemplates -s o --description "Specify options"
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